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Author Topic: simple RF amp using 12AX7 tubes  (Read 7933 times)
RADMANCF
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« on: February 07, 2013, 02:03:29 PM »

Hi, I've just recently started out in amateur radio, and am interested in homebrewing a tube power amp to go with my HT. I have a couple of brand new 12AX7 tubes in my junk box, and was wondering wheather these would be a good starting point.
Thanks,
KD0UFC
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N2EY
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« Reply #1 on: February 07, 2013, 02:04:27 PM »

No

73 de Jim, N2EY
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RADMANCF
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« Reply #2 on: February 07, 2013, 02:08:44 PM »

No

73 de Jim, N2EY
Ok, I realize that the 12AX7 is typically used as an audio preamp tube; is it not suitable for use with RF currents?
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KB1GMX
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« Reply #3 on: February 07, 2013, 03:47:31 PM »

As a power amp your ht puts out more power than they can.  It works fine as a amplifier
but not a high power one.

The idea of a power amp it to use devices that can take the available power and deliver more
not all tubes can do that and 12AX7 is just a lightweight sometimes used in transmitters for
the microphone amplifier or other low level work.


Allison
« Last Edit: February 07, 2013, 03:56:37 PM by KB1GMX » Logged
WB6BYU
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« Reply #4 on: February 07, 2013, 04:21:01 PM »

The 12AX7 will work up to 2m (though not designed specifically for such high
frequencies.)  However, the plate dissipation of each section is 1.2W.  Using both
sides in parallel you might get 3 watts maximum out of the tube
with an optimum circuit.  And it might be difficult to get an optimum circuit
at VHF.

So it only would be practical if your HT was limited to 250mW or less.  However
if you want the experience of building a tube final, it could be done.
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RADMANCF
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« Reply #5 on: February 07, 2013, 08:22:57 PM »

[Tquote author=WB6BYU link=topic=87995.msg652245#msg652245 date=1360282861]
The 12AX7 will work up to 2m (though not designed specifically for such high
frequencies.)  However, the plate dissipation of each section is 1.2W.  Using both
sides in parallel you might get 3 watts maximum out of the tube
with an optimum circuit.  And it might be difficult to get an optimum circuit
at VHF.

So it only would be practical if your HT was limited to 250mW or less.  However
if you want the experience of building a tube final, it could be done.
[/quote]
Ok, that makes sense. Among the reasons I found the 12AX7 to be attractive is the fact that it is widely used in guitar amps, and thus, should be available well into the future. Could somebody suggest a power tube that is common in guitar amps, but suitible for use in RF amps?
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K5LXP
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« Reply #6 on: February 08, 2013, 06:37:19 AM »

Quote from:  link=topic=87995.msg652273#msg652273 date=1360297377
Could somebody suggest a power tube that is common in guitar amps, but suitible for use in RF amps?

I believe the 807 was an audio amp many hams made into power oscillators and low power amps.

Here's an audio amp made from an RF tube:

http://www.wavac-audio.jp/sh833_e.shtml


Mark K5LXP
Albuquerque, MN
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KE3WD
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« Reply #7 on: February 08, 2013, 06:57:13 AM »

Your HT is very likely in the VHF or UHF band or both. 

While there are audio power tubes that can be used in the HF spectrum and RF tubes that can be used in the Audio spectrum, you are not likely to find an easy way to use an Audio tube as an RF Power Amp for the bands your HT operates on. 

Then there is the issue of tuning the tube for each frequency as well.  While it is possible to design a tube driven RF amplifier as a broadband notune amplifier, such designs are problematic and not really the better approach. 

For use with the VHF/UHF HT, a solid state broadband amplifier approach would make much more sense.  But building one may end up costing more in time and components than the cost of one of the manufactured solid state amplifiers marketed for the task. 

If you want to experiment with RF and tubes, time well spent in upgrading the ham license if necessary in order to get HF privileges might be the more appropriate path to take.  At the lower HF frquencies, a lot more can be done with the type of tubes you are talking about. 

The 6L6, a rather common guitar amp output tube, for example, can be used as the final in an HF CW amplifier and yield about 7 to 10 watts of RF on the HF SW bands. 

The world of RF design is a lot different from the world of AF amplifier design.  That said, a good understanding of the AF amp can be a good head start to finding out what it takes to amplify RF with tubes.  "two pi LC" and forward.  (that's jargon, for all you self-inflicted Internet Police...)


73
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WB6BYU
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« Reply #8 on: February 08, 2013, 08:51:14 AM »

Quote from: RADMANCF

Could somebody suggest a power tube that is common in guitar amps, but suitible for use in RF amps?



Actually, tubes suitable for guitar amps are often in high demand and overpriced
compared to RF tubes designed for RF service.  The tube in a well-designed
RF amp should last 20+ years if you take reasonable care of it.  Since many
tubes aren't being made any more, I think you'll get better performance at a
lower price if you choose a good RF tube to start with.

The 6V6 / 6L6 series of tubes were commonly used in HF transmitters, along with
the 6AQ5, etc.  But the performance of many such tubes starts dropping off
around 20 - 30 MHz as the stray inductance of the lead lengths becomes more
critical.  A couple inches of wire from the pin to the cathode is no problem at
audio or lower HF, but it is a significant portion of a 1/4 wave on VHF.

Tubes commonly used in 150 MHz commercial equipment included the 2E26 or
6146 (and relatives such as the 6883, 8032, etc.)  Even these couldn't be run
at full ratings compared to what they could do at HF.  One of the better choices
might be a 4CX250 - it is designed for VHF/UHF operation, there are lots of them
around, and you can let it loaf because it is capable of far more output than you
need.  (You can get 200+ watts out from one tube.)  You can find a number of
power amplifier circuits designed for this tube.
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RADMANCF
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« Reply #9 on: February 08, 2013, 09:31:49 AM »

Well, thanks for the suggestions. I had no idea that the tubes commonly used by guitarists were considered to be over priced. The 12AX7s I have, I got in 2007 for about $15 each from guitar center (the big chain guitar shop, for the unfamiliar), and all the power tubes were about $40.
KD0UFC
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W9GB
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« Reply #10 on: February 08, 2013, 10:05:55 AM »

Quote from: KD0UFC
I had no idea that the tubes commonly used by guitarists were considered to be over priced. The 12AX7s I have, I got in 2007 for about $15 each from Guitar Center (the big chain guitar shop, for the unfamiliar), and all the power tubes were about $40.
Yes, many radio amateurs are aware of the Guitar Center retail chain.

I remember, not that long ago (early 1980s), when the 12AX7 sold for $2 or less as NOS / surplus.
The glass vacuum tube production in USA began to close down in early 1980s.  
Jack Welch, new CEO of GE (known as Neutron Jack in that era) shutdown the GE/RCA tube production as well as much of the GE manufacturing (or shipped offshore) by end of 1980s in favor of GE Capital (finance) and Broadcast (NBC) divisions.


RCA's HAM TIPS (from the Golden Age)
http://n4trb.com/AmateurRadio/RCA_Ham_Tips/rca_ham_tips.htm

Most of the glass vacuum tubes, that are now imported to USA, are mfg. in Russia, China, old Balkan countries, Czech Republic, or Slovakia.  The markup by these importers is substantial.  
New Sensor Corp. in Queens (NYC) has locked up ($$) much of the USA distribution for minature audio / guitar tubes from these mfg.
http://www.newsensor.com/

A new Chinese mfg. 807 tube (late 1930s RCA design) now costs > $30.
http://w5jgv.com/downloads/RCA%20807.pdf

The reason that the AUDIO / GUITAR Vacuum tube market exists is due to professional musicians ($$$), who invested their profits to keep American component mfg. (parts) in business to mfg. the parts for classic guitar amplifiers, such as Marshall, Fender, etc.
The Eastern Asian market (10x population of USA) has maintained the audiophile vacuum tube market.
===
FOR VACUUM TUBE RADIOS -- Look at the Classic American Mfg. from 1940 to 1970 period, as well as GE and MOTOROLA for VHF/UHF Land Mobile radios.

RCA Notes for 807 in 1939
http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips0202.pdf

===
Only GLASS TUBES still mfg. that can be used for higher power HF amplifiers are:  811A, 572B, and 3-500Z.
« Last Edit: February 08, 2013, 10:35:04 AM by W9GB » Logged
KA5N
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« Reply #11 on: February 08, 2013, 11:13:00 AM »


     "YOU can't make a silk purse out of a sows ear,  especially if sows ears are not
       sold anymore,"

       A restatement of an old addage.  Just substitute RF tubes usable for low power
       VHF/ UHF  amplifiers  for sows ears.    Tubes that might be considered are the
       2E26 and maybe 5763.    Audio tubes are generally poor choices, what is good
       for a guitar amp just won't do the job for VHF/UHF duty. 
       Then there are all the other parts and circuitry.  Also if you use tubes you will
        need power supplies for the filament power and high voltages.   Power supplies
        will be a problem for portable operation.  Battery power would require some sort   
        sort of convertor. 
        A small amp using transistors is much easier to accomplish and less costly.

       Allen KA5N


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KD0REQ
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« Reply #12 on: February 08, 2013, 11:45:29 AM »

807 was the 6L6 with the plate moved out of the stem and up to a cap on top of the bulb.  that got the frequency band of the tube past 20 meters.
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RADMANCF
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« Reply #13 on: February 08, 2013, 11:58:09 AM »

[quote author=W9GB link=topic=87995.msg652367#msg652367 date=1360346755
Snip
The reason that the AUDIO / GUITAR Vacuum tube market exists is due to professional musicians ($$$), who invested their profits to keep American component mfg. (parts) in business to mfg. the parts for classic guitar amplifiers, such as Marshall, Fender, etc.
The Eastern Asian market (10x population of USA) has maintained the audiophile vacuum tube market.
[/quote]
It's not just classic guitar amps that generate the demand for tubes in the music world. Marshall, Fender, Vox, Peavey, Mesa et. al. keep developing tube amps, and reissuing the old models from time to time. I remember when Vox reissued the AC-30 a few years ago, IIRC it uses a mass of 12AX7s. There are also a number of stompboxes that use tubes these days also. Anyway, I'm actually surprised at the markups on tubes. About how much do they cost to produce? And I suppose service life in a guitar amp isn't a good indicator of service life in RF applications, since the normal operating conditions in a guitar amp would be considered abuse by most other standards (making the tubes clip 'cuz it sounds good), so typically, how long do tubes last in RF power amps, and do they start to sound different as they age?
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KE3WD
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« Reply #14 on: February 08, 2013, 04:31:24 PM »

In modern manufacturing, it isn't how much it costs to produce, it is how many units one can produce and sell. 

When a market drops off, the single price of a unit must rise.  Supply and demand. 

When the majority of customers had tube driven gear, whether radios, tvs, audio, or what have you, the sheer proliferation of devices meant huge sales in vacuum tubes.  So much so that even drugstores used to carry them.  12AX7's once sold for less than a dollar each. 

Today the vacuum tube is a niche market, regardless of what that market may be. 

73
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